Spectacular can be summarized as such: Writing Effective Lightweight Use Cases that Don't Suck :)
In a nutshell, Spectacular is an automated acceptence test-driven development (ATDD) / behavior-driven development (BDD) tool that uses lightweight Use Cases as its Executable Specification.
Spectacular follows ATDD/BDD best practices. Specs live in source control as plain-text files, written in the business user's language. Use case flows can be further broken out into re-usable Action steps, which are connected to the System-Under-Test (SUT) by fixtures written in Groovy.
Since Fixtures are written in Groovy and executed on the JVM, you can call your SUT directly, or via Selenium/WebDriver!